Pulse lengthening circuit



C. W. JOHNSTONE ET AL Sept. 22, 1953 INVENTORS CHARLES w. JOHNSTONEHA-ROLD v. HANCE ATTORNEY Patented Sept. 22, 1953 PULSE LENGTHENINGCIRCUIT Charles W. Johnstone, Garden City, N. Y., and Harold V. Hance,Washington, D. 0., assignors to the United States of America asrepresented by the Secretary of the Navy Application June 6, 1946,Serial No. 674,768

Claims. (Cl. 250-27) This invention relates generally to electroniccircuits for stretching pulse time'duration and particularly pulsestretching circuits requiring no recovery time between responses andproducin negligible time delay between input and response pulses.

In radar, radio, television, and other electronic fields it isfrequently desirable to respond to actuating signals with voltage pulsesof a definite and recognizable duration greater than the actuatingsignal. However, known circuits are limited to responses from individualpulses relatively well separated and will give spurious response to apulse received before the circuit has recovered from the previous pulse.This constitutes a limiting factor where it is desired that a circuitrespond to a plurality of pulse sources having no time relation such asbeacon equipment or to multiple pulse groups such as might be used inidentification systems.

It is, therefore, an object of this invention to provide pulsestretching means which will respond to pulses applied at any timeincluding pulses applied before the termination of a previous pulse orbefore the termination of the stretching of a previous pulse. I

It is another object of this invention to provide pulse stretching meanswhose response begins with the leading edge of the actuating pulse andwhich will respond to pulses applied at any time including pulsesapplied before the termination of a previous pulse or before thetermination of the stretching of a previous pulse.

It is another object of this invention to provide pulse stretching meanswherein the degree of stretch is the same for every response butcontains means for controlling said degree of stretch.

It is still another object of this invention to provide a pulsestretching circuit which may be so adjusted as to be relativelyinsensitive to random noise.

Other objects and advantages of the invention will be apparent from thefollowing description and accompanying drawings in which similarcharacters of reference indicate similar items.

Referring now to the drawings:

Fig. 1 is a schematic diagram of one embodiment of this invention;

Fig. 2 is a series of waveforms illustrating the stretching of a pulsethat is applied to the circuit in Fig. 1;

Fig. 3 shows the changes wrought in the waveform of Fig. 2 by a secondpulse closely following the first.'

Briefly, this invention comprises, in part, a regenerative triggercircuit which is so arranged as to have a stable state and an unstablestate. In quiescence the circuit remains in the stable state untilactuated by an incoming signal, whereupon it will regeneratively driveinto its unstable state and remain in such state for a period dependingupon a suitable time constant circuit which is incorporated therein.During the existence of the unstable state the circuit will produce anoutput pulse whose duration equal that of the unstable state. Means,including the time constant circuit, are added for applying the pulse tobe stretched to the trigger circuit so that upon application of the samethe trigger circuit will drive into its unstable state, or if it wasalready in its unstable state at the time of such application, it willso remain for the same period it would have remained unstable had itbeen in its stable state when the pulse was applied.

The invention may be better understood by reference to Fig. l in detail.The regenerative trigger circuit in the preferred embodiment hereinshown is a direct coupled multivibrator comprising a pair of triodetubes 8 and 9 with their cathodes tied together and connected to groundthrough a common biasing resistor 25. The grid of the first tube 3receives a high bias by its connection through a high resistance 20 to asliding tap on a bleeder circuit l8 and i9 connected between 3+ andground. The second tube 9 of the multivibrator receives its grid biasfrom the bleeder circuit 2!, 23, and 26 which is proportioned so thatthe stable state of the multivibrator exists when tube 8 is conductingand tube 9 is non-conducting. Resistor 2! in addition to serving as anelement in the bleeder arrangement carries the plate load for the firsttube 8 and therefore varies the grid bias on tube 9 in accordance withcurrent variations in tube 8. Resistance 23 serves a second purpose ofisolating the plate of tube 8 from the grid of tube 9. This resistor isshunted by a capacitance 22 to accelerate the reaction of tube 9 tochanges occurring at tube 8. The output pulse of the circuit is takenfrom across the plate load resistance 24 of tube 9 so that uponapplication, for instance, of a negative signal to the grid of tube :3the circuit will regenerate in the usual multivibrator manner to producea negative pulse at the plate of tube 9. The duration of this pulse, fora given setting of potentiometer I8, is primarily controlled by the timeconstant comprising grid limiting resistance 20 of tube 8 and a suitablefixed capacitance l4, connected between the grid of tube 8 and. ground.By so connecting capacitance i l in the multivibrator, the circuit maybe keyed into operation merely by discharging the capacitance 14. Uponthe discharge of capacitor it, the multivibrator will regenerate to itsunstable state where tube 9 conducts and tube 8 cuts off, and willremain in this state until capacitor Hi recharges through resistance 29to a point Where tube 3 commences to conduct. At this instant themultivibrator will again regenerate to its stable state to therebyterminate .the negas tive output pulse at the plate of tube 9'.

In normal operation it is desireditos provide capacitor I l with a rapiddischarge'in response: For this purpose a diode type tube is here shownconnected in series with the path carrying the incomingsignal.

to an incoming pulse signal.

pulse (negative in this case) to the grid of tube 8.

This diode is arranged with its plate side/at the grid of tube 8 andcondenser is so that a negative pulse at its cathode willimmediatelydischarge condenser Ill. shouldhave a quiescent biasequal toor greater than the grid of tube 8 unless it is desired to use same asan upper limit ior'grid biasv and As here shown the cathode of the diodei5 is biased byits connection to the cathode of a cathode follower tube5. Saidchargeon condenser ill.

tube 5 is arranged so that its quiescent current causes apvoltage dropacross its cathode resistor 8? suitable for the requirements of saiddiode.

With the arrangement as described, the amplitude'oi the negative signalpulse will determine:

the discharge level of: said condenser. Hi, and

therefore will also control the chargingtime oi condenser I 4.

As thus far described, theduration of the-out put pulse will depend uponthe duration and am plitude of the input pulse. Furthermore, an inputpulse applied duringthe operation of thecircuit from a previous inputpulse will further discharge the condenser to thereby extend the outputpulse not only in relation to the amplitude and duration of the secondinput pulse but also the time relation of the two input pulses. It isusually desired i to employ the circuit to extend the duration of anyinput pulse by a predetermined amount regardless of the time relation ofthe'input pulses; To this end the condenser must not be allowedtodischargebelow a suitable level. Such a limit is herein provided bytying the cathode of a second diode l to condenser i l at the side itconnects the grid of tube 8. The plate of the second diode The cathode:of diod-e 6- 'i is tied to a tap on a bleeder circuit i5,- i6,- and i1.potentiometer l5 to thedesired discharge limit The voltage on saidplatecshould: be set byfor condenser it. Any condenser voltage-more 4negativethan that prescribed will becarriedoii through said diode. Thesetting of the potentiometer iii controlling the dis-charge limit isoneextent which the condenser must be recharged to fire the tube 8 at theconstant charging rate.

This may be easily accomplished in the.

means of varying the amount of stretch applied to the input pulsesbecause it determines the.

If .this. cathode biasis greaterthanthegrid biason the flrst..7

discharged. to .thesame level T31 as before.

multivibrator tube 8, only input pulses of greater amplitude than thedifference in said bias voltages will be passed by the series diode 6 todischarge the condenser M. The noise clipping level of the circuit maybe adjusted by using a potentiometer l3 to supply the grid bias to thecathode follower'itube 5;"

Referringnow to the explanatory waveforms in Fig. 2, waveform 3irepresents a single negative pulse as it would appear at the input tothecircuit in Fig. 1 or at the cathode of the first'tube 51 The baseline 3'. is also the quiescent charge level of the capacitor Hi assumingan appropriate'setting; of 1 the bias resistor 93. The horizontalbroken? line 3! represents the condenserdischarge' -limit provided bythe second diode] and. the .tap on the bleeder resistor 15. In waveform3i which represents the same pulse as it :w-illllappear at the capacitorI4, the limiting action of said second diode may be noted. Theverticallinesrcommon to the three-waveforms are used to. indicate theirtime relations. It is seen therefore in waveform 3 I', that thecapacitorbegins 'to" charge at the termination of the pulse.

The dottedz horizontal .line 38 represents-the cut on point for thefirst multivibrator tube 8 thus demonstrating that-the negative pulsecuts off said tube with. its leading .edge and renders same conductingagain v:when the capacitor .has nearly regainedits.quiescentchargeuDistance 4i representsc-theduration. of. the pulse and 39 the chargingperiod. required-to fire the tube- 8.

Wavefo'rmiiZ is the outputpulseat-the plate of thesecondmultivibratortube 9f It will-benoted that its timeduration canthesumof-the-original pulse .lengthA l and. thechargingperiod 3t.

InFig. 3 the eiiect .of a pairofcloselygrouped-l pulses-.(waveiorm 33)-upon the circuit in Fig. 1 is demonstrated. As in Fligv 2,. the verticallines common to the waveforms are usedto indicate.

their. time relation. In waveform. 35;" which represents the-voltage. ofcapacitor ldjiti is seen that. the .second pulse arrives before thecondenser has recoveredffr'om thefirst. pulse but' that it "is Thecharging time 39 "is now the same as it. was for a single .pulse .(39 inFig. 2).. The output pulse waveformtfi," has no discontinuity. betweenor because ofTsaidYpairof'pulsbs and'the secondpulse Will" cause:saidcoutput pulse toconti'nue for its normal iduration- M)" as measuredfrom the leading .edgeof said 'secondpulse.

Althoughwehave shown and de'scrib'e'd only,

limitedandspecific embodiments of. the present ,invention, it is to beunderstood that we are fully aware ofithe many modifications possiblethereof.. Therefore, this invention is not to be limited except insofaras is necessitated by the spirit of the prior .art and the scope or" theappended claims.

We claim: 1. A'pulse stretching circuit comprising; a pulse generatingtrigger circuit, ,a normallyl charged capacitor connecte'din parallelwith the input ,to said trigger circuit to initiate an output pulseduring an incoming pulse, and'clampingmeans connected to said capacitorand biasedtolimit the discharge level thereof."

2. A pulse stretching circuit comprising; 'a'pulse generating triggercircuit, a normally charged capacitor connected in parallel with theinput to said trigger circuit to initiate an output pulse coincidentwith the discharge thereof and to terminate the output pulse after apredetermined charging thereof, a charging path for said capacitordetermining the charging rate thereof, a first diode means connected tosaid capacitor, means responsive to and during an incoming signal todischarge said capacitor through said first diode means, and seconddiode means connected to said capacitor and biased to limit thedischarge level thereof.

3. A pulse stretching circuit comprising, a pulse generating triggercircuit, a normally charged capacitor connected in parallel with theinput to said trigger circuit to initiate an output pulse coincidentwith the discharge thereof and to terminate the output pulse after apredetermined charging thereof, a charging path for said capacitordetermining the charging rate thereof, a first diode means connected tosaid capacitor and biased to limit the charging level thereof, meansresponsive to and during an incoming signal to unbias said first diodeand discharge said capacitor therethrough, and second diode meansconnected to said capacitor and biased to limit the discharge levelthereof.

4. A pulse stretching circuit comprising, a pair of vacuum tubesconnected together in a regenerative manner so'as to form a pulsegenerating trigger circuit having one stable state and one unstablestate, a normally charged capacitor connected in parallel with the inputof the one of said pair of tubes which is conducting during the stablestate, a charging path for said capacitor determining the charging ratethereof, a first diode means connected to said capacitor and biased tolimit the charging level thereof, means responsive to and during anincoming signal to unbias said first diode and discharge said capacitortherethrough, and second diode means connected to said capacitor andbiased to limit the discharge level thereof.

5. A pulse stretching circuit comprising, a pair of vacuum tubesconnected together in a regenerative manner so as to form a pulsegenerating trigger circuit having one stable state and one unstablestate, a normally charged capacitor connected in parallel with the inputof the one of said pair of tubes which is conducting during the tablestate, a charging path for said capacitor determining the charging ratethereof, and a rapid discharge circuit connected to said capacitor todischarge same in response to and during an incoming signal.

CHARLES W. JOHNSTONE. HAROLD V. HANCEi.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,241,256 Gould May 6, 1941 2,350,069 Schrader et a1 May 30,1944 2,405,843 Moe Aug. 13, 1946 2,411,573 Holst et a1. Nov. 26, 19462,419,340 Easton' Apr. 22, 1947 2,489,824 Shenk Nov. 29, 1949 2,589,085Houghton Mar. 11, 1952

